Gentlemen, please avert your eyes.
OK. Are they gone?
Girls....the party hats are tiny little band-aid type devices that often have little metal beebees in them. They are affixed to the nipples as markers for the radiologist, since sometimes nipples can appear as abnormalities on the mammogram. I think they also use them for moles or other areas that they want to mark to avoid a mis-reading of the picture?
Sorry for the very non-technical explanation. It's all I could find on the internets.
And sorry for saying nipples twice (uh, I mean three times) in this post.